The present invention relates generally to radiometers and more particularly, to an aircraft or orbiting spacecraft radiometer including an array of electromagnetic energy-to-electric signal transducers that are simultaneously responsive to energy from a terrestrial or atmospheric region, which signals are combined phase-coherently to attain high resolution information or images of the region.
Orbiting spacecraft have been employed as platforms for deriving terrestrial and atmospheric geophysical parameters in response to microwave and millimeter wave energy derived from terrestrial and/or atmospheric regions. An electromagnetic wave-to-electric signal transducer, in the form of an antenna or an optical energy detector, derives a replica of the radiant energy given off by the terrestrial region; the replica usually has noise-like properties.
Important properties of such radiometers are the spatial and temperature resolutions. The spatial resolution of a radiometer is the precision with which the radiometer can distinguish signals arising from adjacent locations, and is dependent directly on the transducer size so that large transducers (antennae) are required for high spatial resolution. Temperature resolution, i.e., sensitivity, is the precision with which a radiometer can distinguish incident radiation intensity levels. For certain applications, it is desirable to attain very great spatial resolution, as well as a high temperature resolution.
In the prior art, spatial resolutions on the order of 1 degree of arc in each plane have been attained by utilizing transducers including relatively large parabolic reflectors or arrays. Theoretically, higher spatial resolution can be obtained by increasing the parabolic reflector diameter; however, increasing the diameter of the reflector causes several problems. In particular, there is a loss of temperature resolution with increased reflector diameter due to the reduced dwell time per resolution cell. When an array is used, there is an additional sensitivity loss due to lower efficiency and narrower band width of the large array. A wide band width radiometer is important because of the wide band width, noise-like properties of the radiation emanating from the region of interest. In order to realize high spatial resolution, it is necessary to employ a detector having a relatively short integration time per spatial resolution element, so that no spatial information is lost in the detection process, but this degrades the temperature resolution. Attempts to increase band width of arrays by the use of special phase shifters and to gain integration time by the use of multiple beam forming networks have been found to cause lower transducer efficiency, with no net overall increase in sensitivity.